Mass spectrometry systems are analytical systems used for quantitative and qualitative determination of the compositions of materials, which include chemical mixtures and biological samples. In general, a mass spectrometry system uses an ion source to produce electrically charged particles (e.g., molecular or polyatomic ions) from the material to be analyzed. Once produced, the electrically charged particles are introduced to the mass spectrometer and separated by a mass analyzer based on their respective mass-to-charge ratios. The abundance of the separated electrically charged particles are then detected and a mass spectrum of the material is produced. The mass spectrum provides information about the mass-to-charge ratio of a particular compound in a mixture sample and, in some cases, information about the molecular structure of that component in the mixture.
For determining molecular weight of a compound, mass spectrometry systems employing a single mass analyzer are widely used. These analyzers include a quadrupole (Q) mass analyzer, a time-of-flight (TOF) mass analyzer, ion trap (IT-MS), and etc. For more complicated molecular structure analysis, however, tandem mass spectrometers (Tandem-MS or MS/MS) are often needed. Tandem mass analyzers typically consist of two mass analyzers of the same or of different types, for instance TOF-TOF MS or Q-TOF MS. In a tandem MS analysis, ionized particles are sent to the first mass analyzer and an ion of particular interest is selected. The selected ion is typically transmitted to a collision cell where the selected ion is fragmented. The fragment ions are transmitted to the second mass analyzer for mass analysis. The fragmentation pattern obtained from the second mass analyzer can be used to determine the structure of the corresponding molecules.
For example, in a triple quadrupole (QQQ) mass spectrometer an ionization source produces a plurality of parent ions. The first quadrupole is used as a mass analyzer to select a particular parent ion. Then, the selected parent ion is dissociated into daughter ions in the second quadrupole via photodissociation and/or collisionally induced dissociation. Subsequently, the third quadrupole is used as a mass analyzer to separate the daughter ions based on their respective mass-to-charge ratios. The resulting mass spectrum can be used to identify the daughter ions, which can be useful in identifying the structure of the selected parent ion.
In the example described above, the second quadrupole can be used as a collision cell to facilitate collision induced dissociation of the selected parent ion. In such a collision cell, the selected parent ions are sent into an RF quadrupole field which is pressurized up to approximately 1 to 10 mbar with a background gas (normally an inert gas such as argon). When the parent ions collide with the background gas, a portion of the translation energy of the parent ions is converted into activation energy that is sufficiently high to break certain molecular bonds to form daughter ions. The RF quadrupole field facilitates confinement of the daughter ions and the remaining parent ions until further mass analysis. The fragment pattern produced characterizes the original molecule and provides information about its structure.
In combination with other ion optic elements, an RF quadrupole can also be used as an ion trap for storage of ions. A potential gradient is formed along the axis of the quadrupole, and ions are trapped in a potential well. The ion trapping provides a possibility for performing ion accumulation, charge reduction, and ion-ion chemistry. In some tandem mass spectroscopy applications, an ion collision cell/linear ion trap is also used as a mass selective device. A molecular ion of a given mass is selected, isolated, and stored. Ion-gas collisions and/or ion-ion reactions may also be performed.
When the quadrupole is used as a linear ion trap or as a collision cell, specific potential distributions are formed along the axis of the quadrupole. In a linear ion trap, a potential well is formed for confining ions (which may be either positively or negatively charged). The potential well typically is formed by using a quadrupole with gate electrodes at each end of the quadrupole. Holding the gate electrodes at a relatively “high” potential (at “trapping potential”) and the quadrupole at a relatively “low” potential provides the potential well that confines the ions. In a collision cell, a potential gradient is necessary for accelerating ions along the axis of the quadrupole. This potential distribution is typically formed by using an evenly segmented quadrupole and applying a DC potential gradient to the different segments of the quadrupole. Opening the potential well by lowering the potential at the exit gate electrode allows ions to be released from the linear ion trap; lowering the exit gate electrode potential for a short period of time and then returning the exit gate electrode to the “trapping potential” releases a short burst of ions (an “ion packet”). The ion packet may be directed towards another component of the mass spectrometer, such as a mass analyzer and/or a detector.
Manipulation of ions in a mass spectrometer is dependent upon the controlled application of specific RF and/or DC potentials to components of the mass spectrometer, e.g. applying potentials to a quadrupole, applying gate potentials, or applying suitable potentials in a TOF mass analyzer. What is needed is an apparatus which provides for the needed RF and/or DC potential distributions needed for manipulating ions in a mass spectrometer.